1. Field of the Invention
The present invention relates to motor vehicle bodywork parts, whether they be static parts or opening members.
2. Description of the Related Art
Most bodywork parts are made of metal sheet, usually steel sheet. Bodywork parts made of plastics material are becoming more and more widespread, such as bumpers, tailgates, fenders, etc., which parts coexist at their interfaces with other parts that are still made of metal, such as fenders, hoods, doors, etc.
Such parts made of plastics material may present defects that are particularly visible in an outside panel because the panel can be seen from the outside, and defects show up in particular at the periphery of such an outside panel.
Various ways are known for accommodating differences of expansion under the effect of temperature, in particular when hot because of strong sunlight.
Thus, it is known to select the component materials of such parts depending on their coefficients of linear thermal expansion (CLTE). It is also known to make such parts, while providing them with reinforcement seeking to avoid excessive deformation. Thus, such parts are dimensioned, in particular in terms of thickness or in the form of ribs, or indeed by fitting them with reinforcement made of stronger material, so as to improve the stiffness of the parts under consideration.
It is also known to handle defects in local manner, i.e. at the edge of a part where excessive changes in clearance or departures from flush alignments are most visible.
There is only a limited choice for the plastics materials from which to make such parts. Material choice depends on a technical and economic compromise seeking to comply with various constraining clauses of specifications for the vehicle. It frequently happens under certain conditions laid down in the specification, in particular extreme levels of sunlight, that these parts deform excessively, thereby degrading appearance, and in particular degrading the perceived quality of clearances and alignments that ought to be flush. Panels that are strongly deformed can also give rise to jamming and scratching on painted parts facing such panels, particularly if they are included in a moving system.
In order to overcome those defects, designers are thus constrained to overdimension components or a set of assembled-together components so as to make them less deformable, which then makes them heavier and more expensive, which therefore goes against the initial objective for choosing to make them out of plastics material.
Thus, for tailgates, particularly those that are said to be “all plastic”, the components are made up of two main parts, a box situated on the inside of the vehicle and a panel situated on the outside of the vehicle. The box and the panel are assembled together, typically by adhesive. They are also connected to the body and they are subjected to various forces via hinges, actuators, a lock, and a peripheral weather strip. Those various forces vary depending on varying situations, in particular depending on whether the tailgate is closed, open, being subjected to high temperatures, or indeed being slammed shut in particularly rough manner. The panel is often painted and is the part that is visible when the tailgate is closed, and it is therefore with the panel that the above-mentioned defects are observed the most easily, particularly in comparison with surrounding bodywork elements. Under the effect of temperature, the panel suffers both from a loss of performance, i.e. it becomes more deformable, and from expansion of the materials from which it is made.
The loss of mechanical performance leads to deformation because the tailgate, even when it is in the closed position, is subjected to forces simultaneously by the actuators for assisting opening, by the weather strip compressed against the body, and by the hinge and anchoring points relative to the vehicle such as the hinges, the lock, and the tailgate stops on the vehicle body. Deformation is particularly severe at the hinges, since they are typically made of steel and secured to the body, thereby constituting points where the tailgate is constrained. The tailgate can deform only in zones where it is not constrained, e.g. between the hinges or beyond them, i.e. at the margins of the part. This mechanical deformation is also associated with deformation due to expansion. Expansion occurs within the panel, and also within the box, and indeed in differential manner between those two components, since they may be made of different plastics materials, the box typically having a coefficient of linear thermal expansion that is smaller than that of the panel. The connections between the panel and the box, particularly when they are adhesive connections, are therefore subjected to forces that are particularly intense.
In order to validate a tailgate design, in particular in terms of high temperature performance, the specifications generally require tests to be performed that consist in imposing successive temperature variation cycles on the tailgate, or more particularly on the assembly constituted by the outer panel and the box or inner panel of the tailgate. Those cycles have an amplitude of about 80° C., but that varies between manufacturers, and the number of cycles performed lies in a range of 15 to 20 cycles, approximately. In that type of testing, the tailgate does not always have enough time to return to its initial shape prior to expansion before the next cycle begins. An effect of deformation accumulating from cycle to cycle is therefore observed with the deformation of the tailgate becoming progressively more degraded.